Detrital Influences on Soil Carbon Stabilization in Wet Temperate Forest Soils

Soils, with their potential to store and stabilize carbon (C), are an essential resource for sustaining forest productivity, as well as for efforts to reduce atmospheric C concentrations. Protecting existing soil C and harnessing the sequestration potential of our soils require an improved understanding of the processes through which soil organic matter accumulates in natural systems. Currently, competing hypotheses exist regarding the dominant mechanisms for soil C stabilization. Many of our long-standing hypotheses revolve around an assumed positive relationship between the quantity of organic inputs and soil C accumulation, while more recent hypotheses have shifted attention towards the more complex controls of microbial processing and organo-mineral complexations. The Detrital Input and Removal Treatment (DIRT) experiment allows us to test these competing hypotheses through direct field manipulations of above and belowground detrital inputs. The following dissertation presents findings of soil C response to twenty years of detrital manipulations in the H.J. Andrews Experimental Forest, located in the Cascade Mountains of the Pacific Northwest U.S. Annual additions of low quality (high C:N) wood litter to the soil surface led to an apparent positive, yet highly variable change
in accumulation of soil C, while the separate, equal magnitude addition of higher quality (low C:N) needle litter had no significant effect on soil C over the twenty year study period. Additions of low quality (high C:N) wood litter led to a substantial accumulation of light fraction soil C. The observed contrast in soil C response between these two addition treatments demonstrates the role of litter quality in regards to soil C accumulation and suggests quality may be of greater relevance than litter quantity in certain forest environments. The detrital input reduction treatments, including the removal of belowground root inputs and the aboveground removal of surface litter, had no effect on bulk soil C concentrations over the twenty year study period. Further, the combined removal of both above and belowground organic matter inputs also had no effect on soil C, yet did show greater potential for driving soil C loss than the individual reduction treatments. Following the removal of live roots, concentrations of heavy fraction soil C increased, potentially due to reduced priming activity associated with the decline of the rhizosphere community. Above and belowground detrital reduction treatments caused a decline in soil solution DOC at 30 cm. The addition of wood debris led to a sharp increase in 30 cm soil DOC, while in contrast, the addition of needle litter led to a decline in DOC at the same soil depth, likely due to microbial priming. These diverse findings strongly support modern soil C stabilization hypotheses which emphasize the strength of control microbial activity and mineral stabilization have on soil C accumulation. The insights gained from these studies provide important avenues for improved land management practices to promote soil C accumulation, as well as an improved mechanistic understanding required for advances in earth system models.